Final answer:
The correct answer is C. HF, which is a weak acid. The starting pH given in the question suggests that the unknown acid is not a strong acid like HCl, as a strong acid would have a much lower initial pH.
Step-by-step explanation:
The question is asking to identify which among the given compounds is most likely the unknown acid that has a starting pH of 3.5 when titrated with a strong base, NaOH. We are given that the unknown is an acid because of its initial pH value. The options are NaF, LiCl, HF, and HCl.
Among these, LiCl is a salt and not an acid, and NaF is also a salt resulting from a weak acid (HF) and a strong base (NaOH). Looking at the other two choices, HF is a weak acid, whereas HCl is a strong acid.
By examining titration curves, it can be seen that the titration of a strong acid with a strong base will have an equivalence point at a pH of 7.00, whereas the titration of a weak acid with a strong base will have an equivalence point above pH 7, around pH 8.72.
Since the unknown has an acidic pH of 3.5, it is unlikely to be a strong acid like HCl, as the starting pH would be much lower. Consequently, HF, a known weak acid, fits the profile of having a higher initial pH and is the correct answer.